In an analog FM (frequency modulation) stereo radio system, the left channel (L) and right channel (R) of the audio signal are conveyed in a mid-side (M/S) representation, i.e. as mid channel (M) and side channel (S). The mid channel M corresponds to a sum signal of L and R, e.g. M=(L+R)/2, and the side channel S corresponds to a difference signal of L and R, e.g. S=(L−R)/2. For transmission, the side channel S is modulated onto a 38 kHz suppressed carrier and added to the baseband mid signal M to form a backwards-compatible stereo multiplex signal. This multiplex baseband signal is then used to modulate the HF (high frequency) carrier of the FM transmitter, typically operating in the range between 87.5 to 108 MHz.
When reception quality decreases (i.e. the signal-to-noise ratio over the radio channel decreases), the S channel typically suffers more during transmission than the M channel. In many FM receiver implementations, the S channel is muted when the reception conditions gets too noisy. This means that the receiver falls back from stereo to mono in case of a poor HF radio signal (typically referred to as a mono dropout).
Even in case the mid signal M is of acceptable quality, the side signal S may be noisy and thus can severely degrade the overall audio quality when being mixed in the left and right channels of the output signal (which are derived e.g. according to L=M+S and R=M−S). When a side signal S has only poor to intermediate quality, there are two options: either the receiver chooses accepting the noise associated with the side signal S and outputs a real stereo signal comprising a noisy left and right signal, or the receiver drops the side signal S and falls back to mono.
Parametric Stereo (PS) coding is a technique from the field of very low bitrate audio coding. PS allows encoding a 2-channel stereo audio signal as a mono downmix signal in combination with additional PS side information, i.e. the PS parameters. The mono downmix signal is obtained as a combination of both channels of the stereo signal. The PS parameters enable the PS decoder to reconstruct a stereo signal from the mono downmix signal and the PS side information. Typically, the PS parameters are time- and frequency-variant, and the PS processing in the PS decoder is typically carried out in a hybrid filterbank domain incorporating a plurality of Quadrature Mirror Filter (QMF) banks.
It has been proposed in WO2011/029570, PCT/EP2011/064077 and PCT/EP2011/064084 to use PS encoding of a received FM stereo signal in order to reduce the noise comprised within the received FM stereo signal. The general principle of the Parametric Stereo (PS) based FM stereo radio noise reduction technology is to use parametric stereo parameters derived from the received FM stereo signal, in order to reduce the noise comprised in the received left and right signals. The disclosure of the above mentioned patent documents is incorporated by reference.